He was poor, and just seems to me to lack the X-factor that a goalkeeper for a top team has.
Credit to him for stopping the Jota volley and the Mane 1 v 1 - but we shouldn't turn a blind eye to the terrible handling which led to Liverpool's 3rd goal. When Salah raced through for the 4th goal, I had no confidence in him even getting near to the shot. Admittedly a goalkeeper is disadvantaged there but we've seen De Gea stop many of those in his time here.
As a point of comparison, De Gea would have been crucified on this forum for the errors Dean made today. Both goalkeepers aren't perfect, but I wouldn't trust Dean to start in the Europa League final over De Gea.